A tungsten halogen lamp used in a lighting unit with a dichroic mirror used for lighting up goods at a shop or the like is proposed as a bulb used in a lighting unit with a reflecting mirror.
As shown in FIG. 3, in a lighting unit with a dichroic mirror, a funnel-shape reflecting mirror 1 and a bulb as a light source, for example, a tungsten halogen lamp 20 are combined into one component with the central axis of the reflecting mirror 1 approximately coinciding with that of the tungsten halogen lamp 20. The reflecting mirror 1 comprises a reflecting part 3 having a reflecting-mirror surface provided with a dichroic film 2 at one end and a neck portion 4 at another end. The neck portion 4 is connected to the reflecting part 3. The tungsten halogen lamp 20 has a straight-tube shape and is sealed at one end. A sealing portion 21 of the tungsten halogen lamp 20 is inserted into the neck portion 4 of the reflecting mirror 1. Further, the sealing portion 21 and the neck portion 4 are inserted into a base 12, and an adhesive 17 is injected into the base 12, thus fixing the parts so as to form one component.
However, when the conventional lighting unit with a dichroic mirror using the tungsten halogen lamp is lighted up with the reflecting mirror 1 positioned downward for many hours, the adhesive 17 injected into the base 12 for fixing the reflecting mirror 1, the tungsten halogen lamp 20, and the base 12 deteriorate. Thus, the adhesive strength decreases, and the reflecting mirror 1 therefore falls out as shown in FIG. 4. As a result, a lighted object can be damaged in some cases. Consequently, there has been a problem in safety.
Even if the adhesive 17 does not deteriorate, the adhesive strength also decreases due to vibration for a long period and therefore the reflecting mirror 1 falls out in some cases.